1. Field of the Invention
The present invention relates to an electrostatic coating apparatus, which coats an annular article such as a stator core or a driving magnet for an electric motor with electrostatically charged powder, and it pertains to a method for electrostatically coating such an annular article. The present invention also relates to a fixture for holding an annular article for such electrostatic powder coating.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 60-193560 discloses an electrostatic coating apparatus for coating an armature of an electric motor with electrostatic powder. The electrostatic coating apparatus comprises a chain conveyor for transporting the core to be coated along a transporting path and an electrostatic spray station disposed half way of the transporting path to apply electrostatically charged powder onto the armature. The apparatus also comprises a fixture for holding or supporting the core. The holding fixture is formed of a pair of conductive metallic cylindrical columns, each cylindrical column having a central hole or bore which is open at one end of the column. Both ends of a holding shaft fixed to the center of the armature are detachably fitted to the central holes, respectively. The armature is transported by the chain conveyor along the transporting path with the armature rotating with the cylindrical columns of the holding fixture. While the armature passes through the electrostatic spray station, electrostatically charged powder of solid particulated material are ejected onto the armature to coat the armature with the powder.
However, the aforementioned conventional coating apparatus cannot coat the portion of the armature covered by the holding fixture because the powder cannot reach there, while the exposed portions outside of the holding fixture can be coated. Moreover, since the holding fixture is formed to hold the shaft of the armature, a hollow annular article such as a stator core of an outer-rotor-type spindle motor cannot be held by the holding fixture of the conventional apparatus. When the holding shaft is fitted in the inner diameter of the core, the core can be held thereby. However, the interior surface of the core cannot be coated.
Furthermore, it may be sometimes desirable that the thickness of the coating is changed at portions of the core. As disclosed in Japanese Laid-Open Patent Application No. 10-145988, for example, when a stator of a motor opposes a permanent magnet ring with the stator having coils wound around core sections, it is required that the thickness of the coating on the core is made different, complying with the demand for each portion thereof. For example, the coating on the surface of the core opposing the permanent magnet should be thinner than that on a coil winding portion and interior surface of a mounting hole of the core. In the apparatus disclosed in that patent application, the core held by a particular holding fixture is coated two times, with the second coating being applied after a first coating is removed for some portion where the thickness of the coating is required to be thinner. The portion having one layer of the coating (single coated portion) film and the portion having two layers thereof (double coated portion) are thereby formed on a core such that it has different thicknesses of the coating for the different portions of the core. However, since the prior art does not teach how to hold an annular core during the coating such that the entire surface of the core may be coated, it is likely that the coating is uneven in some portions and un-coated in other portions.